The detection of structural flaws in components that are designed for incorporation into larger structures such as machines, buildings, or engines is important for ensuring the quality, safety, and operation of such structures. In particular, the detection of structural flaws in cylindrical titanium billets, which are ultimately incorporated into aircraft engine disks, is an important endeavor in the aerospace industry. For example, structural flaws or defects in billets such as voids, cracks, holes, discontinuities, and inclusions may be detrimental to the operation of aircraft engine disks and the aircraft as a whole if appropriate measures are not taken to remove the flawed region of the billet. Also, in the case of titanium billets, it is necessary to downgrade a heat of material if a hard-alpha inclusion is found so that it is not used as prime rotor material for a jet engine application. In order to address such concerns, non-destructive testing techniques such as ultrasonic testing (UT) have been employed to detect the size and position of structural flaws in billets without affecting the structural integrity of the billet in any way. When flaws are detected above a pre-defined threshold value, the flawed portion of the billet may be excised to ensure that only the highest quality material is incorporated into aircraft engine components.
The UT technique employs a probe carrying a transducer element that sends a high frequency ultrasonic beam through a test material. The transducer element is typically a piezoelectric material that converts electrical pulses into ultrasonic sound waves and vice versa. When the ultrasonic beam comes into contact with a structural defect, ultrasonic waves, referred to as ‘echoes’, are reflected back to the transducer element. The transducer element subsequently converts any reflected echo signal into electrical signals that are sent to a computer controlled instrument. The computer translates any received electrical signals into a readable data display, known as a c-scan, and reports information relating the size and location of any structural defects to the operator.
A disadvantage of conventional UT employing a single transducer element is apparent during the inspection of cylindrically-shaped billets, however. Billet imperfections, such as circumferential out-of-roundness or bowing, may cause the probe to tilt away from the central axis of the billet, such that portions of the billet center may miss inspection entirely.
The phased array ultrasonic inspection technique is an advanced variation of the conventional UT inspection technique and overcomes some of its inherent limitations. Phased array ultrasonic testing employs an array of multiple transducer elements in a single probe unit. The individual transducer elements in the array can be separately pulsed to transmit ultrasonic waves according to programmed time-delays. The pulsed ultrasonic waves interact constructively or destructively to create a predictable primary wavefront, or ultrasonic beam, that travels through the test material. By the use of focal laws that contain time delays for firing the individual elements of the transducer, it is possible to strategically time the firing of the pulsed ultrasonic waves from the multiple transducer elements in order to shape the beam, steer or sweep the beam through angles, and/or vary the depth of the beam focus through the test material. Software called a focal law calculator establishes the specific time delays for firing each of the transducer elements required to produce the desired angle for beam steering, the desired depth of focus, and/or the desired beam shape for probing the test material.
During phased array inspection, any reflected echo signals arising from structural defects are received by the transducer elements in the array and are converted to electrical signals that are sent to a computer. The computer displays the amplitude of reflected echo signals and reports the position of the defect to the operator. Echo information may be displayed as a c-scan image which provides a two-dimensional representation of the material as a flattened or planar view.
The ability of the phased array technique to permit beam steering or beam sweeping through angles greatly enhances the detection of flaws in cylindrical billets compared with conventional UT, as it ensures that the entire cylindrical volume of interest, including the billet center, is inspected even if the probe is tilted away from the central axis.
One limitation of the ultrasonic testing method for the non-destructive testing of titanium billets is the high number of false detections that can arise from background noise due, in part, to the granular structure of titanium. Furthermore, due to the mechanics of rotating billets weighing thousands of pounds as well as limitations due to finite increments of data collection used during the scan process, it should be assumed that when a flaw is detected it may not have been detected optimally. Therefore, echo signals that fall below a pre-established rejection threshold should be identified for closer evaluation by the inspector performing the ultrasonic test.
A system is needed to evaluate phased array echo data from billets by accounting for background noise in order to improve the reliability of structural defect signals and reduce or eliminate false detections. Moreover, a system is needed to improve the evaluation of echo signals obtained during automated phased array measurements.